The present invention relates to carbon black pellets having good dispersibility and a high pellet hardness, and to a process for the production thereof.
In the processing of industrial carbon blacks it is preferred to use granulated products, which are frequently called carbon black granules, carbon black pellets or pelletized carbon black. At present, two different processes are used for the large-scale granulation or pelletization of carbon blacks: wet pelletization in a pelletizing machine with subsequent drying, and dry pelletization in a pelletizing drum. The two processes have markedly different process parameters, which are closely connected with the physical processes in the agglomeration in each case and with the resulting pellet properties.
The pelletizing machines used for wet granulation are granulators having a shaft with pins. They comprise a stationary tube (also called a "stator" below) which is arranged in a horizontal position and has a shaft with pins which rotates therein. The pelletizing chamber available for the purpose of granulation is located between the axis of the shaft and the wall of the tube. In the pelletizing chamber, the carbon black is conveyed by the pins of the rotating shaft from the inlet at one end of the tube to the outlet at the other end of the tube, during which agglomeration takes place as the carbon black rolls off the stationary tube wall. The residence time of the carbon black in the pelletizing machine may be lengthened by providing a baffle plate at the outlet or by raising the outlet relative to the inlet. Typical pelletizing machines have a length of from 1 to 3.5 meters and a diameter of from 200 to 760 mm.
In the pelletizing machine, the powdered industrial carbon black is mixed intimately with water, optionally with the addition of a binder. Depending upon the type of carbon black, in the case of water contents of from 40 to 60 wt. %, based on the total weight, spherical carbon black pellets are obtained. The moist pellets are subsequently dried in a further process step.
In the case of wet pelletization, agglomeration takes place by means of liquid bridges and capillary forces between the carbon black particles. The size of the capillary forces permits comparatively high circumferential speeds of from 10 to 20 m/s at the tips of the pins, producing intimate mixing and the subsequent formation of pellets.
In the case of wet pelletization, the necessary residence time for good formation of carbon black pellets is in the region of a few seconds, usually less than 20 seconds. The residence time can be lengthened to the range of several minutes by means of a baffle plate or by raising the outlet relative to the inlet.
Without the use of binders, the achievable pellet hardness of the carbon black pellets obtained by means of wet pelletization is in the range of from 0.1 to 0.3 N in the case of pellet diameters of from 1.4 to 1.7 mm. In order to increase the pellet hardness, a soluble, solid substance (pelletizing agents) is added to the pelletizing water and remains in the carbon black pellets after drying. As a result of the intensive wetting of the carbon black and the homogeneous dissolution of the pelletizing agent in the pelletizing water, the pelletizing agent is distributed in optimum manner and is able to fully develop its strengthening action.
For dry pelletization there are used pelletizing drums comprising a rotating tube which is likewise arranged in a horizontal position. The interior of the tube is called the pelletizing chamber. For the purpose of granulation, the powdered industrial carbon black is pre-densified, for example in accordance with DE 38 32 404, and granulated in the pelletizing drum by rolling off the rotating tube wall. The Van-Der-Waals and electrostatic forces responsible for dry pelletization are considerably lower than the capillary forces which are at work in the case of wet pelletization. The granulation must be carried out in a correspondingly gentle manner. The circumferential speed of the pelletizing drum is, therefore, only from 1 to 2 m/s. With markedly higher circumferential speeds, the rolling-off movement does not occur because of the high centrifugal force. Furthermore, the force acting on the pellets that are forming is so high that they are immediately destroyed again. The achievable pellet hardness is usually less than 0.1 N in the case of pellet diameters of from 1.4 to 1.7 mm.
On account of the low Van-Der-Waals forces it is necessary, in order to initiate granulation, to add granulation seeds in the form of pre-densified carbon black to the carbon black. The seeds are introduced into the pelletizing drum before the beginning or are fed in continuously during operation of the pelletizing drum in an amount of from 1 to 30 wt. %, based on the amount of carbon black powder. If granulation seeds are not added, dry granulation may not take place or the pellets may break.
The typical residence time of the carbon black in the dry pelletizing drum is from 1 to 4 hours. In order to ensure an adequate carbon black throughput (in kg/h), dry pelletizing drums must be considerably larger than pelletizing machines for wet pelletization. Dry pelletizing drums having diameters of 2 m and lengths of 18 m are used in production. The filling amounts of such drums are several tons. In contrast, the filling amounts of pelletizing machines are only a few kilograms.
In both wet and dry pelletization it is possible to use additives to increase the pellet hardness and/or improve the dispersibility.
On account of their greater pellet hardness, wet-pelletized carbon blacks generally also have a greater dispersion hardness than dry-granulated carbon blacks. Accordingly, they are used mainly in the rubber industry. The wet-granulated carbon blacks are readily dispersible in the highly viscous rubber compositions. Their high pellet hardness allows them to be transported easily in pneumatic conveyor apparatuses.
Dry-pelletized carbon blacks are used principally as pigments in paints and plastics. However, carbon black powders are still used in large amounts in those fields, and their handling can lead to a high degree of dust irritation in the workplace. That dust irritation can be reduced only by the use of pelletized carbon blacks.
WO 96/01875 describes the use of organic compounds to produce carbon black pellets which are resistant to abrasion and are readily dispersible. Suitable organic compounds are non-polymeric organic compounds, thermoplastic homopolymers, thermoplastic copolymers, or a wax. Those substances are added to the carbon black in amounts of from 10 to 48 wt. % during pelletization. Pelletization may be carried out in pelletizing machines and in pelletizing drums. According to that specification, pelletization takes place in a manner similar to that of wet pelletization by strengthening of the cohesive forces between the carbon black particles as a result of the molten organic substances. Accordingly, when pelletizing machines are used they are operated using the same process parameters as in conventional wet pelletization. According to WO 96/01875, if the organic substances are not added, carbon black pellets are not obtained. In the case of pelletization in a pelletizing machine having a shaft with pins, the minimum amount of organic substances to be added, depending upon the DBP value of the carbon black powder, is approximately 29 wt. % but typically 38.5 wt. %. According to the WO specification, the addition of the organic substances to the carbon black is carried out, for example, by atomization with the aid of a pressure nozzle.
WO 96/21698 describes the wet pelletization of carbon black in a pelletizing machine with the aid of an aqueous solution of an organic binder. In order to remove the water, the carbon black pellets that are formed must be dried at temperatures at which the organic binder does not decompose. The amount of binder contained in the dried carbon black pellets is from 0.1 to 50 wt. %.
An object of the present invention is to improve the properties of carbon black pellets to enhance their usefulness and which can be produced with a low energy consumption.
Another object of the present invention is to increase the hardness of the carbon black pellets while at the same time having good dispersion behavior, and with only a low dust content.